i
l~
I!I
Sky on a Chip: The Fabulous CCD
James Janesick, Caltech, and Morley Blouke, Tektronix, Inc.
The CCD revolution
TELESCOPES
than those in
are no bigger
usetoday
two generations
ago.
Yet, curiously,
astronomers
can now
study objects that are hundreds or even
thousands
of times fainter th;3.ll those
accessible in the 1930's.
• The explanation is simple: the last few
decades have brought an unprecedented
improvement
in the sensitivity
of light
detectors. The changes in our own era are
as significant
as those caused by the
development of the telescope in the 17th
century or the photographic
plate in the
19th century.
Of all the new astronomical
tools, one
in particular
stands out because it is an
almost perfect radiation detector. Known
as a charge-coupled device, or CCD, it has
the ability to register almost every photon
(particle
of light) that strikes it over
wavelengths ranging from the near infrared to X-rays.
In contrast,
the most
sensitive photographic
emulsion used by
astronomers
records two or three percent
of the light striking it, and over a much
narrower range of wavelengths.
came about
indi-
rectly. It began in the late 1960's when
two researchers at Bell Telephone Laboratories, Willard S. Boyle and George E.
Smith, sought to invent a new type of
computer
memory circuit. In particular,
they wanted to construct
an electronic
analog to magnetic bubbles.
While Boyle and Smith conceived the
CCD as a memory element, it rapidly
became apparent
that the tiny chip of
semiconducting
silicon they first demonstrated in 1970 had many other applications, including signal processing and imaging (the latter because silicon responds
to visible light).
In recent years the CCD's initial promise as a memory element has vanished. It
has, however, established
itself as the
premier imaging device for scientific applications,
especially astronomical
ones.
Indeed, CCD's are at the forefront of an
ongoing revolution in all types of electronic
photography.
They are already
common in lightweight video systems and
auto-focus cameras.
This picture of Uranus is thought to be the
first astronomical image made with a
charge-coupled device, or CCD. It was
obtained in 1975 by scientists from the Jet
Propulsion Laboratory and the University
of Arizona, using the 61-inch telescope in
the Santa Catalina Mountains near Tucson.
Recorded at a wavelength of 8900 angstroms in the near infrared, it shows a
region of enhanced methane absorption
(dark area) near Uranus' south pole.
\
PIONEERING
\
USE
\
: \
\
'
\
\
\
\ \
\
Determining the brightness distribution in a celestial object with a charge-coupled device
can be likened to measuring the rainfall at different points in a field with an array of
buckets. Once the rain has ceased, the buckets in each row are moved horizontally across
the field on conveyor belts. As each one reaches the end of the conveyor, it is emptied into
another bucket on a belt that carries it to the metering station where its contents' are
measured. Artwork by Steven Simpson ..
238
l""'""-
Sky
& Telescope,
September, 1987
Astronomers
were among the first to
recognize the extraordinary
imaging capabilities of the CCD. In 1972 researchers at
the Jet Propulsion Laboratory (JPL) established a program to develop CCD's for
space astronomy.
Three years later the
JPL team, working with scientists from
the University of Arizona, took what i
probably the first astronomical
image obtained with a CCp (see the photograph
above).
Today every major telescope
in the
world has a CCD camera as part of its
observing arsenal. Such a device can be
used either for direct imaging or as the
detector in a spectrometer or other instrument. In addition, the Vega, Giotto, and
Suisei probes to Halley's comet, and many
future space projects
including
the
Hubble Space Telescope, Galileo, and the
Advanced X-ray Astrophysics
Facility will have CCD's at the heart of their
camera systems.
The operation of a CCD is quite simple
in principle.
An elegant
analogy
by
Jerome Kristian of the Carnegie Insti-
l••.•••.
I!!I
t
Sky on a Chip: The Fabulous CCD
James Janesick, Caltech, and Morley Blouke, Tektronix, Inc.
TELESCOPES
today
are no bigger
than those in use
two generations
ago.
Yet, curiously, astronomers can now
study objects that are hundreds or even
thousands of times fainter th;lll those
accessible in the 1930's.
, The explanation is simple: the last few
decades have Qrought an unprecedented
improvement in the sensitivity of light
detectors. The changes in our own era are
as significant as those caused by the
development of the telescope in the 17th
century or the photographic plate in the
19th century.
Of all the new astronomical tools, one
in particular stands out because it is an
almost perfect radiation detector. Known
as a charge-coupled device, or CCD, it has
the ability to register almost every photon
(particle of light) that strikes it over
wavelengths ranging from the near infrared to X-rays. In contrast, the most
sensitive photographic emulsion used by
astronomers records two or three percent
of the light striking it, and over a much
narrower range of wavelengths.
The CCD revolution came about indirectly. It began in the late 1960's when
two researchers at Bell Telephone Laboratories, Willard S. Boyle and George E.
Smith, sought to invent a new type of
computer memory circuit. In particular,
they wanted to construct an electronic
analog to magnetic bubbles.
While Boyle and Smith conceived the
CCD as a memory element, it rapidly
became apparent that the tiny chip of
semiconducting silicon they first demonstrated in 1970 had many other applications, including signal processing and imaging (the latter because silicon responds
to visible light).
In recent years the CCD's initial promise as a memory element has vanished. It
has, however, established itself as the
premier imaging device for scientific applications, especially astronomical ones.
Indeed, CCD's are at the forefront of an
ongoing revolution in all types of electronic photography. They are already
common in lightweight video systems and
auto-focus cameras.
This picture of Uranus is thought to be the
first astronomical image made with a
charge-coupled device, or CCD. It was
obtained in 1975 by scientistS from the Jet
Propulsion Laboratory and the University
of Arizona, using the 61-inch telescope in
the Santa Catalina Mountains near Tucson.
Recorded at a wavelength of 8900 angstroms in the near infrared, it shows a
region of enhanced methane absorption
(dark area) near Uranus' south pole.
\
PIONEERING USE
\
\
: \
\
\
\
Determining the brightness distribution in a celestial object with a charge-coupled device
can be likened to measuring the rainfall at different points in a field with an array of
buckets. Once the rain has ceased, the buckets in each row are moved horizontally across
the field on conveyor belts. As each one reaches the end of the conveyor, it is emptied into
. another bucket on a belt that carries it to the metering station where its contents are
measured. Artwork by Steven Simpson.
238
l-.....,
Sky & Telescope, September, 1987
Astronomers were among the first to
recognize the extraordinary imaging capabilities of the CCD. In 1972 researchers at
the Jet Propulsion Laboratory (JPL) established a program to develop CCD's for
space astronomy. Three years later the
JPL team, working with scientists from
the University of Arizona, took what is
probably the first astronomical image obtained with a CCp (see the photograph
above).
Today every major telescope in the
world has a CCD camera as part of its
observing arsenal. Such a device can be
used either for direct imaging or as the
detector in a spectrometer or other instrument. In addition, the Vega, Giotto, and
Suisei probes to Halley's comet, and many
future space projects - including the
Hubble Space Telescope, Galileo, and the
Advanced X-ray Astrophysics Facility will have CCD's at the heart of their
camera systems.
The operation of a CCD is quite simple
in principle. An elegant analogy by
Jerome Kristian of the Carnegie Insti-
should be in such a form that the brightnesses of the objects viewed can easily be
measured with high precision in absolute
units, such as magnitudes.
• Linearity
the output should be
uniform; if one object is twice as bright as
another,
its image should be twice as
strong.
Unlike many other detectors currently
in use, the CCD excels in everyone
of
these areas.
A CCD's resolution is determined primarily by the number of picture elements,
ADVANTAGESOFTHECCD
or pixels, making up the imaging area.
understand why a CCD is so useful
More pixels mean better resolution,
as
powerful,
we need to understand
evident in the photograph at right.
.:;2: a good astronomical
detector must
The first large image-forming
CCD's
=.::. ::s most important requirements
are:
were introduced by Fairchild Semiconduc• 31gh resolution - it must be able to
tor in 1973. They contained 10,000 pixels
_<='; ::Ine detail.
arranged in 100 rows and 100 columns. A
• :-ligh quantum efficiency - as much
short time later, RCA Corp. made ver.. E :l:.e radiation
falling on the sensor as
sions achieving
the standard
television
;;cssible must be actually detected.
resolution
of 512 rows by 320 columns
• Broad spectral response - the detec- . (163,840 pixels). Detectors
like this are
--;- ;::lust be sensitive to radiation over a
typical of the tCD's now in use at
observatories around the world.
~ :a.nge of wavelengths.
• Low noise - any spurious signals it
The CCD's made especially for Galileo
:rates must be far weaker than the
and the Hubble Space Telescope by Texas
=..=s:ial signals it is trying to measure.
Instruments
are now becoming available
to astronomers in limited numbers. These
• L<uge dynamic range - the differhave 640,000 pixels arranged in an 800-by~,~ between the brightest
and faintest
::==crs it can view simultaneously must be
800 array. Texas Instruments
has also
made chips with over one million picture
~ ~t
as possible.
elements for the cameras
of the pro• Photometric
accuracy - its output
of Washington
is often used to
-=::s...-ribehow it works (see the bottom
-·;o.=ram on the facing page). Imagine an
of buckets covering a field. After a
.-'<corm,
the buckets are sent by con=:Dr belts to a metering station where the
-.-,:mnt of water in each is measured.
~,:;,== a computer
could display a picture
':.~ :::ow much rain fell on each part of the
-::K.. In a CCD system, the "raindrops"
~hotons. (See the box on page 240 for
~e technical description.)
........ion
=y
=
l=gest charge-coupled device currently available is made by Tektronix, Inc.
}rising an array of 2,048 rows and 2,048 columns of light-sensitive picture elements,
- drip measures over 2.5 inches square. All illustrations supplied by the authors unless
- otherwise.
This image of the planetary
nebula
NGC 6781 in Aquila was made by combining three CCD images, each made with a
different color filter. Like all CCD images,
it is built up from an array of tiny squares
(inset), each representing the data collected
by one pixel. The more pixels that make up
the image, the finer the detail visible. This
image was obtained with a CCD having
512 rows and 320 columns of picture
elements. Courtesy Rudolph Schild.
posed High-Resolution
Solar Observatory.
Today the largest CCD is made by
Tektronix,
Inc. (see left and next page)
and incorporates
over 4,000,000
pixels
(2,048 by 2,048). Its active image area
measures over 2.5 inches across, making it
the world's largest integrated circuit.
Impressive
as these numbers
sound,
they are small compared to the size and
resolution
of photographic
emulsions.
Even the coarsest-grain
35-millimeter film
might have a resolution
equivalent
to
25,000,000 pixels. Fine-grained astronomical plates can do a lot better.
However, CCD images have other ad-.
vantages that render them far superior to
photographs.
For example, the bottom
left diagram on page 241 shows how the
quantum
efficiency
of a typical CCD
exceeds that of several other detectors
used by astronomers.
The eye, the first astronomical
"detector," has a quantum efficiency of about
one percent. In other words, of every 100
September, 1987, Sky & Telescope
239
photons entering the pupil, only one is
detected. In contrast, for every 100 photons hitting a CCD, 50 to 70 or even more
are registered. In addition, the range of
wavelengths to which the eye can respond
is much smaller than the CCD's.
The
same limitation is true of photocathodes,
film, and vidicon-type television detectors.
. It has recently been shown that the next
generation of CCD's will respond usefully
from I angstrom
in the "soft"
X-ray
region of the spectrum to greater than
10,000 angstroms in the near-infrared
(see
the box on page 242). This huge wavelength span is far greater than that of any
other detector known; it is one of the
properties
that makes CCD's useful for
such diverse missions as the Advanced
X-ray Astrophysics
Facility and Hubble
Space Telescope.
I • I
••
I:
I1;·:1
I..
: IIII
,
11= :
••
r::
!
I
1I
How a Charge-Coupled Device Works
1nTTT:TTI.··1
•• =1
Electrons
~
I
Output amplifier
HO
I:·:
A Tektronix 2,048-by2,048-pixel charge-couplet:
device shown life size.
This monster chip, the
world's largest, is currently under development
for astronomical applications. Also shown here fur
size is the 800-by-800-pix,
CCD (inset) made especially by Texas Instruments for the Hubble
Space Telescope and the
Calileo Jupiter probe.
While perfect examples
are worth dozens of times
their weight in gold, cosmetically defective models
have already been made
available to the astronomical community.
1
~1PiX~I-
~
•••
~
I
The operation of a CCD is illustrated schematically by this simplified chip consisting of
nine pixels, an output register, and an amplifier. Every pixel is subdivided into three
regions, or gates; each is an electrode whose voltage can be varied. Left: During an
exposure the central gate of each pixel is "on" (yellow areas) and its neighbors are "off"
(green areas). This creates "electron buckets" under the middle electrodes and barriers
between adjacent pixels. Right: At the end of the exposure the gate voltages are changed
and electrons shifted one gate to the right as new potential wells are created and old ones
are destroyed.
•
•
~
• •• ••
••~ •
••
·I:I~)
· I I"
•• •
1m)
I
I
I
••
1
~•
•• ••
I
~
•
••
I
Left: As the voltages are cycled again, electrons flow.from the right-most gate of one pixel
to the left-most gate of its neighbor. The electrons in the right-hand pixels are transferred
to the output register. Right: Before the pixel array can be shifted again, charge must be
transferred, one pixel at a time, through the output register and amplifier. When this
register has been completely emptied, another cycle of pixel-array transfers is executed.
These steps continue in a systematic way until every charge packet has moved horizontally
along its row, vertically down the output register, and into the amplifier, where it is
measured. In a real astronomical CCD, this process can take as long as IO seconds.
240
Sky
& Telescope,
September,
1987
A charge-coupled
device must perform
four tasks to generate an image. These
operations are:
• Photoelectron
generation (rain).
• Electron collection (buckets) .
• Charge transfer (conveyor belts) .
• Readout (metering station).
The first operation relies on a physical
process known as the photoelectric effect
when light hits certain materials,
electrons are generated. When a pattern
of light from, say, a distant galaxy falls
on a CCD, the photons that make up
the incoming image are absorbed within
the chip's silicon lattice, where they give
rise to free electrons.
In the second step these so-called
photoelectrons
are collected in the nearest discrete collecting sites, or pixels
(picture elements). These collection sites
are defined by an array of electrodes,
called gates, formed on the surface of
the CCD. In our rainfall analogy, the
buckets represent the pixels .
The third stage, charge transfer,
is
accomplished
by changing the voltages
on each gate in a systematic way so that
the electrons
move horizontally
from
one pixel to the next, conveyor-belt
fashion .
At the end of each row is the so-called
output
register, which is in effect a
conveyor running
perpendicular
to all
the others. It transports the charge packets one by one to the output amplifier
where, in the final operating steps, the
electrons are counted and converted to a
form suitable for storage in a computer,
display on a television
monitor,
and
analysis by astronomers.
But a high quantum
efficiency
and
broad spectral response are useless if the
detector
introduces
false signals
that
swamp those from a faint astronomical
source. The electrons produced in each
pixel of a CCD are counted by a single
transistor
built into the chip's output
circuitry. The accuracy of the counting is
determined by the noise introduced by the
transistor. Today's CCD's have amazingly
low noise and can reliably detect charge
packets containing as few as 10 electrons
(generated
by the capture of 15 to 20
visible-light photons).
Small as these numbers seem, astronomy . demands
even greater
sensitivity.
Counting errors are typically comparable
to the number of photons collected every
second when a 24th-magnitude
object is
viewed with a 6O-inch telescope.
Substantial effort is now being expended to
enable CCD's to count charge packets
containing fewer than four electrons.
One of the most significant advantages
a CCD has over other detectors is its large
dynamic range. For CCD's this important
property is defined as the ratio of the
largest charge packet that can be measured to the smallest..
The larger the size of a pixel, the more
electrons it can hold before overflowing.
For example, the 15-micron-square
pixels
on Texas Instruments'
800-by-800 CCD
can each hold about 75,000 electrons.
Picture elements twice as wide have a
capacity
of nearly 1,000,000 electrons.
These numbers reveal that current CCD's
can have dynamic ranges up to 100,000
(1,000,000 electrons in a full pixel divided
by 10 electrons of noise).
. Large dynamic range is particularly important for an astronomical
detector be"'" 100
~
"
B
>o<:
Oi
10
\J.J
i3
;;-
;;:
!OJ
:;;;
2<:
"
0; 0.1
2.000
4,000
6,000
8,000
10,000
WAVELENGTH (angstroms)
The superiority of the charge-coupled
device (CCD) over almost all other imageforming detectors is illustrated here. In the
red and near-infrared portions of the spectrum, CCD's are more efficient at detecting
radiation than any other type of device. In
the ultraviolet, however, the performance
of older CCD's illustrated here is much
worse than television-type detectors and
even photographic film.
100,000
-J 10,000
~
C9
i/j 1,000
f-::J
:::
is
100
10
RELATIVE EXPOSURE -
CCD's are exceptionally linear; doubling
the exposure yields a signal exactly twice as
strong. This holds over an enormous exposure range, enabling CCD's to detect simultaneously very bright and very faint objects
in a single image. This behavior contrasts
strikingly with that of photographic emulsions, which are only linear over a narrow
range of exposure. Emulsions must also
overcome a threshold before they start to
form an image at all. Worse, beyond a
certain point emulsions saturate; their ability to detect light actually decreases if the
exposure is too long.
cause of the large brightness differences
commonly found between, say, the faint
outer parts of galaxies and their brilliant
nuclei. The greater the dynamic range, the
greater the CCD's ability to image both
bright and dim objects in 'the same field.
In stark contrast, the best photographic
plates can reproduce a brightness range of
only 100 to 1.
MORE THAN
PRETTY
PICTURES
Even the most sensitive detector is essentially worthless to astronomers
if it can't
be used to determine reliably the brightness of what is being observed in some
sort of absolute units such as magnitudes.
This requires that the detector be stable
over long periods of time so that it always
produces the same output signal for a
given light input.
Because they are solid-state
devices;
CCD's are inherently stable. Once a chip
has been calibrated by observing standard
stars, photometric
measurements
made
during the course of a night can be held to
an absolute accuracy of 0.5 percent (equivalent to an uncertainty
of about 0.005
magnitude).
The CCD also exhibits remarkable linearity. In other words, the number
of
electrons collected in a pixel is precisely
proportional
to the number of incident
photons (see the diagram above). This is
in sharp contrast to photographic
film
and vidicon-type television detectors. The
former has a very complicated response to
This simulated "raw" image from a chargecoupled device does not give a realistic
representation of the well-known Blackeye galaxy, M64 in Coma Berenices, because individual pixels have varying sensitivities to light. This effect is shown here
schematically by dividing the -image into nine regions of grossly different sensitivities. Images such as this are obviously
not very useful to astronomers or anybody
else.
To overcome this non uniformity problem,
observers use a technique called flat-fielding. During an observing run an exposure is made of a uniform source such as
the dawn sky or inside of the telescope
dome to map the variations among the
CCD's pixels. This image represents the
flat-field frame- corresponding to the simulated raw image of the Black-eye galaxy
shown above.
j"
Once the response of each pixel is known,
observers can scale the data by an amount
determined from the flat-field exposure and
thereby correct for each pixel's individual
response. The result is the same as that
which would be registered by a CCD with
pixels of identical sensitivity; it can now be
analyzed by astronomers. Image 'courtesy
Rud~lph Schild.
September, 1987, Sky
&: Te1esro:;re
The All-Seeing CCD
The CCD operates over an enormous
wavelength
range, but this versatility
does not come easily. When
illuminated from the front, it is rather
insensitive to ultraviolet and X-ray radiation. The pixel electrodes ("gates")
are opaque to these photons.
A 20-percent improvement
in shortwavelength response can be made by
coating the front surface with a phosphor that converts incident ultraviolet
radiation to longer wavelengths where
the device is more efficient. A better
method involves thinning the CCD and
illuminating it from the back. Photons
can then enter the active region unimpeded by the gates.
Further
treatment
of the thinned
CCD is required because photoelectrons can get trapped by a small "back
side potential well" at the chip's rear
surface.
Researchers
have developed
two ways to "push"
these electrons
toward the front-surface electrodes.
In one approach,
called back side
charging,
the CCD is flooded
with
intense ultraviolet radiation. This generates excess photoelectrons
that destroy
the well and charge the rear surface
negatively as long as the chip is kept
cold. The CCD's on the Hubble Space
Telescope will employ this technique.
The "flash gate" is a more permanent solution. An electrical potential
generated
between the CCD's silicon
and a thin layer of platinum or gold
deposited on its back side eliminates
the unwanted well. Second-generation
instruments
for the Space Telescope
will probably use this technology.
PHOTON ENERGY (electron volts)
10,000 1,000
100
10
100
t'
<J)
\
{J
80
Q;
-2,
>-
~ 60
\
\\
'-
lU
<3
-Measured
--Expected
40
fE
lU
~
;:: 20
<
<l:
X-ray
::::.
o
Extreme
ultraviolet
0
1
10
100
1,000
10,000
WAVELENGTH (angstroms)
Recent technological advances enable
charge-coupled devices to respond to
radiation across the spectrum from
near-infrared to soft-X-ray wavelengths.
No other detector so efficiently responds
over such a broad spectral range.
242
Sky & Telescope,
September, 1987
determined
by directly
measuring
the
light and at best can achieve a photoamount of charge generated.
This raises
metric accuracy of only 5 percent over a
an interesting possibility: in a single image
small portion of its already narrow dy(actually, many very short exposures supernamIC range.
imposed), a CCD can provide an X-ray
Vidicons are also highly nonlinear. The
astronomer
with simultaneous
high-resorelationship
between
their
input
and
and
spectroscopic
data. No
lution
spatial
output signals must be approximated
by a
complex mathematical
expression that is , other detector can do this.
different for each point on the detector.
CCD's FOR EVERYBODY?
This makes the extraction of photometric
Even though CCD's are almost perfect
information
difficult and time consuming.
detectors,
they are not suitable for all
Even when the required procedures
are
astroilOmers.
They are very expensive; a
followed, vidicon photometry is only accu2,048-by-2,048-pixel
chip presently
lists
rate to about two percent.
for over $80,000, or 19 cents per pixel!
In contrast,
CCD's are linear 'to 0.1
Devices with standard TV resolution range
percent over most of their dynamic range,
from $2,000 to $10,000 (1.2 to 6 cents per
far superior to any other device. This
pixel), depending on their quality.
makes it relatively simple to remove the
CCD's
generally
remain
beyond the
effects of pixel-to-pixel variations in sensireach of most amateurs,
though the intivity. In practice
this is achieved by
creasing popularity
and decreasing price
means of a technique
known as "flatof
lightweight
video
cameras is changing
fielding" (illustrated on page 241).
this somewhat. Several Japanese manufacVidicons
and related
detectors
have
turers sell CCD's for about $200 each.
other serious flaws that limit their astro(Cosmetic rejects go for about half this
nomical usefulness. They all form images
price.) But even chips this cheap are far
on a light-sensitive
medium
that
is
more expensive than photographic
film.
scanned by a beam of electrons just as in
Perhaps the most important
disadvana television camera. The first problem
tage of CCD's is the amount of ancillary
encountered is to know exactly where the
electronics needed to make them work.
beam is at any instant, for its position
Prime
among these accessories is a powercan't be measured directly. Thus the relaful computer to analyze the tremendous
tive positions of points in the image may
amount of data these tiny silicon chips
not correspond
exactly to those in the
create. A single image from a 2,048-byobject viewed.
2,048
CCD will contain 10 megabytes of
Compounding
these positional
uncerinformation.
The amount of data gathtainties are the distortions introduced by
ered during a single observing run could
the magnetic
fields used to focus the
rapidly grow to one billion bytes or more.
readout beam. Taken together, these efThis must be stored, calibrated, and anafects compromise the fidelity with which
lyzed - a very expensive enterprise.
vidicons reproduce
a given scene. These
Lastly, a CCD, like a racing car, reproblems do not apply to a CCD. The
quires a full-time
engineering
support
position of each pixel is fixed rigidly when
team if it is to work at peak performance.
the chip is manufactured.
Nevertheless,
an increasing number of
BEYOND THE VISIBLE
amateur
astronomers
are taking
the
plunge and experimenting
with CCD's
In the past decade more and more
(S&T: January, 1985, page 71, and April,
astronomers have turned their attention to
1986, page 407). This trend is sure to
those parts of the electromagnetic
speccontinue as CCD's and home computers
trum invisible to our eyes. Increasingly,
become cheaper and more powerful. For
observations are being made at ultraviolet
amateurs
and professionals
alike, the
(1200-3500 angstroms), extreme ultraviolet
CCD is truly a dream coming true. With
(120-1200 angstroms), and soft-X-ray (1.2performance
improving continually
it is
120 angstroms) wavelengths.
difficult to imagine what further impact it
Photons in these short-wavelength
portions of the spectrum are more energetic
will
havein the
on the
com~~
munity
next astronomical
decade.
than their visible counterparts.
When absorbed in the CCD's silicon lattice these
James Janesick is head of the advanced
high-energy photons generate many elecimaging sensors group at the Jet Propulsion
trons, the exact number depending on the
Laboratory. He has developed CCD's for
wavelength (energy) of the radiation. For
NASA space missions and ground-based astronexample, a 2.1-angstrom
X-ray photon
omy for the last 14 years. Morley Blouke is
will spawn, on average, 1,620 electrons.
principal scientist in the CCD engineering
For wavelengths
less than 100 anggroup at Tektronix, Inc. He is currently involved in designing, fabricating, and testing
stroms, therefore, a single photon can be
large CCD's for scientific applications.
detected and its energy (or wavelength)